Jb. Peng et al., PHASE-TRANSITIONS IN LANGMUIR-BLODGETT-FILMS OF CADMIUM STEARATE - GRAZING-INCIDENCE X-RAY-DIFFRACTION STUDIES, Langmuir, 13(6), 1997, pp. 1602-1606
Variations in the structure of multilayer Langmuir-Blodgett (LB) films
of cadmium stearate with temperature have been monitored by grazing i
ncidence synchrotron X-ray diffraction. At room temperature the film s
tructure comprises a centered rectangular unit cell in the plane of th
e substrate with the hydrocarbon chains normal to the substrate surfac
e. As the temperature increases up to 90 degrees C, the diffraction pa
ttern due to this in-plane structure becomes diffuse and the intensity
decreases. Changes in the Q-values of the in-plane spots indicate an
anisotropic expansion of the unit cell. At around 100 degrees C the fi
lm undergoes a phase transition from a layered three-dimensional cryst
alline structure to an array of rod structures packed hexagonally. The
rods are oriented parallel to the surface of the film. The core of ea
ch rod is 6.7 Angstrom in diameter and appears to be composed of cadmi
um cations and the carboxylate groups of the fatty acid chains. The al
iphatic chains are randomly arranged around these rods. The rod struct
ure is retained on cooling, but the observed correlation length is red
uced.